使用新穎式設計架構之壓控電容可調式帶通濾波器設計

Abstract

本論文目標為研發新式壓控電容調制之可重構式帶通濾波器，藉由調整壓控電容之對應電容，能在更大範圍內控制中心頻率或通帶頻寬，並可於微波及射頻通訊應用中多功能操作。為達成此一目標，本研究提出兩種新式設計方案：第一種設計方案以兩組串聯形式之共振殘段電路與一組並聯形式之壓控共振電路串接，藉著恰當調整壓控共振電路之高低通帶兩側，有效縮放合成頻寬，並可進一步使用串聯電路之反共振特性，壓抑並聯壓控電路產生之諧波。第二個設計方案則在各共振器間引入可調制之耦合係數，並於輸入與輸出端加入可調整外部品質因數之壓控式阻抗轉換器，以同時達到阻抗匹配，有效解決過去使用固定式阻抗轉換器之重構式帶通濾波器共有的輸入端與輸出端阻抗不匹配問題。兩種新式設計方案均可用以協助設計可重構式帶通濾波器之雛形。 藉助此二新式設計方案，本研究實現了四種不同的重構式帶通濾波器。第一個設計採用第一種設計方案，依循較簡單傳統的設計方法，設計並實作出可重構式帶通濾波器。量測到的介入損失低於2 分貝，於高低頻通帶兩側分別擁有21.8%及18.2%的調制能力，並可有效壓抑其通帶諧波至5.5 GHz。第二個設計以準集總元件實作出略有改變的新建構單元，同樣使用一般設計方法，完成一個輕巧，於1 GHz 擁有相當大通帶變化比例（五比一）的頻寬可調式微帶線帶通濾波器。同時，藉由恰當控制電路中的混合電磁耦合係數，可輕易控制相對比例頻寬。另外，採用可調制的阻抗轉換器，使頻寬變化過程中仍能達成輸出端及輸入端之阻抗匹配。第三個設計提出一個具有可切換高低側傳輸零點、擁有高頻率及高頻寬調整性之可重構式雙模帶通濾波器。藉由簡單的T 型微帶線結構與壓控電容調制，應用略有修改的第二式設計方案導入濾波器雛型，並使用疊代方式合成所需頻率響應。不論傳輸零點位於通帶之高側或低側，實作完成之電路的1 分貝頻寬於中心頻率750 至1240 MHz 中均可達到50 至175 MHz 的頻寬變化，並保有2.9 分貝以下之介入損失。 最後，同時以略有修改之第二種設計方案及疊代方法為基礎，使用壓控電容分別調整環狀雙模共振器之奇偶模共振頻率，實現中心頻率及通帶頻寬均能控制之濾波器。藉著簡單的分析雛形與量測驗證，所設計的可重構式帶通濾波器具有33%之中心頻率調整比例、優良的頻寬變化率，以及多樣式的通帶重構型式，例如典型巴特沃斯(Butterworth)、柴比雪夫(Chebyshev)，甚至全拒(all-rejected)式等的響應特性。

This dissertation aims to design novel varactor-tuned reconfigurable microstrip bandpass filters (BPFs) that are capable of being adjusted to different center frequencies or passband widths for multi-functional operations in microwave and radio frequency communication systems. To achieve this goal, we propose two novel design schemes: The first design scheme is a conceptual structure. It consists of a cascade of two series stub-based structures in shunt with a varactor-tuned microwave resonators to adjust the bandwidth, and realizing passband edges tunabilities. The second design scheme is developed by including a tunable coupling coefficient for inter-resonator coupling tuning as well as a pair of controllable immittance inverters at the I/O ports for an external quality factor. Hence, this scheme can be modified slightly according to the design specification, but its general flow is kept. The two novel design schemes can facilitate shaping prototypes, and guide the design of reconfigurable BPFs. By applying these novel design schemes when necessary, we designed four reconfigurable BPFs with advanced responses: The first design applies the former design scheme along with a simple ordinary procedure to design and fabricate a BPF with a low passband insertion loss less than 2 dB, a 21.8 % higher band edge tuning range, a 18.2 % lower band edge tuning range, as well as wideband suppression greater than 17 dB at frequencies up to 5.5 GHz. The second design adopts a quasi-lumped circuit topology and a varactor-based mixed electric and magnetic coupling mechanism, in which the second design scheme is used in designing a prototype. The BPF implemented for demonstration is a compact microstrip BPF of a very high passband bandwidth tuning ratio (5-to-1 ratio) centered at 1 GHz. The third design chooses a simple T-shaped dual-mode resonator which is developed according to a slight modification on the second design scheme and is externally coupled and perturbed with varactor diodes. In addition, an iterative procedure corresponding to this modified scheme is also presented to guide the design. The implemented BPF has a 1-dB bandwidth adjustable from 55 to 175 MHz for center frequency ranging from 750 to 1240 MHz with a low insertion loss less than 2.9 dB. The final design uses a simple loop-shaped dual-mode resonator tapped and perturbed with varactor diodes, in which the modified second design scheme and the proposed iterative procedure are also exploited in developing the BPF design. The fabricated microwave BPF is with an up to 33% center frequency tuning range, an excellent bandwidth tuning capability, as well as a freedom to change the filter response, such as the ordinary Butterworth, Chebyshev, and all-reject specifications.